The present invention relates to improvements in the nondestructive testing of structural materials, and more particularly, the invention relates to an ultrasonic test head and transducer.
Ultrasonic test equipment for structure parts such as pipes or the like, usually require a particular orientation of such parts relative to the test transducer, particularly if a part moves while it is being examined. Moreover, the equipment requires calibration in order to permit interpretation of any signal being picked up. Particularly, the test equipment includes ultrasonic transducers coupled in a suitable manner to the test object and launch test signals into that object. The latter will return echo signals which have different significance. Legitimate boundaries as well as internal inhomogeneities, flaws, cracks, etc., all may produce echos. The transit time as well as the amplitude of such echos constitute informative parameters which require interpretation. The transducer operation, the signal producing electronics as well as the signal receiving and processing electronics, contribute to the wave shape of the signals as ultimately received and evaluated. It is, therefore, necessary to calibrate the transducers. For this, it has been necessary in the past to couple the transducer to a reference object or dummy preferably having dimensions, contour and acoustic properties equal to or at least closely matching the corresponding features of the test objects. The reference object is not only used for purposes of signal level calibration, gain adjustment, sensitivity and threshold adjustments, etc., but also for purposes of determining the requisite angles of incidence for the test beams, the skip distance in the case of testing pipes or plates, and the requisite actual distance needed for a specific task.
The use of such calibration objects was found to be a problem in some instances. Take, for example, the following situation. It appears that a best suited reference object is a test object known, e.g. to have no defects. It may be the first or one of the first objects made, or a portion of an object made in a first production run, being a part of a pilot series, etc. The thus chosen object is now provided with a definite "defect" to be used as a reference in the calibration procedure. It was found that this defect simulation is not sufficiently accurate nor are such simulated errors accurately reproducible if different reference objects are made. Another point is the delay incurred when the calibration object is taken from the production which must be halted until the calibration has been completed.
A test procedure has been suggested (U.S. Pat. No. 4,106,326, which eliminates the disadvantages outlined above by using a particular reference object or dummy in a separate calibration installation to ascertain operating parameters for the test electronics as well as the test heads, simulating true test conditions. A particular test standard or reference element (to be distinguished from the reference object or dummy) is then used in the calibration installation as adjusted to generate particular responses. That standard is then temporarily installed in the on-line test equipment and its electronics is then adjusted on the basis of the responses one obtains with the standard in comparison with the responses the same standard involved in the calibration electronics. This procedure operates quite satisfactorily, particularly because the calibration can be made ahead of production runs. A standard or reference element to be used for that purpose is disclosed in U.S. Patent application Ser. No. 878,240, filed Feb. 16, 1978. Utilization of this reference element was found to be suitable and reliable. Nevertheless, the reference element is used in conjunction with calibrating procedure that must precede test and production runs.